As a technology to generate image data on the interior of a subject using acoustic waves, an ultrasonic echo method and photoacoustic tomography (PAT) method, for example, are known. The ultrasonic echo method is a method for generating image data by transmitting ultrasonic waves to the interior of the subject, and receiving the ultrasonic waves reflected by the tissue inside the subject. The photoacoustic tomography method is a method for generating image data by receiving acoustic waves irradiated from the interior of the subject by the photoacoustic effect. The photoacoustic effect is a phenomenon whereby acoustic waves (typically ultrasonic waves) are generated when such electromagnetic pulses as visible lights, near-infrared lights and microwaves are irradiated onto a subject, and the subject is heated due to the absorption of the electromagnetic waves, and is thermally expanded.
A time-domain method and Fourier-domain method are known as beam forming methods for image forming using the ultrasonic echo method and PAT method. The Fourier-domain method is a method for Fourier-transforming time-series signals so as to transform variables from time to frequency by transforming the spatial coordinates into a wave number, and then performing processing (NPL (Non-Patent Literature) 1). The Fourier-domain method is characterized in high-speed processing by using a fast Fourier transform (FFT). Since CPU speeds have increased, software-based numerical processing has become practical, and because of recent advancements in parallel processing, the future of software-based image processing, that is the Fourier-domain method, is promising. Particularly in a PAT image forming apparatus, it is preferable to use a Fourier-domain method.    [NPL 1] Minghua Xu and Lihong V. Wang: Photoacoustic imaging in biomedicine, Review of Scientific Instruments, Vol. 77, 041101, Apr. 17, 2006